1. Field of the Invention
The present invention relates generally to automotive transmissions and more specifically to an arrangement for automatic transmissions which enables the beginning of a shift operation to be determined with accuracy.
2. Description of the Prior Art
Shifting between gears of a multi-forward speed type transmission is achieved by selectively supplying line pressure to a given friction element or elements.
In order to control the shifting it has been proposed to use two solenoid valves to the control the selective supply of line pressure to the friction elements. The decision as to which shift is necessary is based on the engine load and vehicle speed while the timing of the shift is based on the ratio of the transmission input shaft and output shaft rotational speeds.
However, during shifting a finite time is required from the time that a shift instruction signal is generated and the time the line pressure is actually supplied to the friction elements, and the shift is actually initiated. Accordingly, in order to obviate the detrimental effect of the delay between the issuance of the shift command and the actual implementation of the shift, on the control of the transmission, it is necessary to detect just when the shift actually begins to occur.
In this connection it has been proposed, by way of example, to control the amount of torque delivered to the transmission from the engine via the torque converter by controlling the amount of torque converter slip during shifts based on the shift initiation timing.
FIG. 4 is a timing chart which shows the relationship which develops between the engine rotational speed, the transmission output torque and the manner in which the friction element engaging line pressure develops.
As will be appreciated from this drawing, at time point t0 a shift command is issued, and at time t1 the spool valve arrangement responds to the command and the level of the hydraulic pressure which is fed to the friction element or elements for the given shift which begins to rise. At time t2 the pressure reaches a level whereat sufficient engagement of the friction elements is induced to permit power to be transmitted between the input and output shafts and thus marks the actual initiation of the shift operation. Immediately following time t2 the engine rotational speed Ne undergoes a marked change and the restrains the change in output torque.
JP-A-55-69738 discloses an example of an arrangement which seeks to determine the actual shift initiation following a shift command. In this arrangement the rate of change of the engine speed is monitored. When this rate exceeds a predetermined level, shift initiation is deemed to have occurred.
However, with this arrangement, it is necessary to constantly monitor the engine speed until the actual shift initiation. Viz., it is necessary to sample the engine speed during the period t0-t1 during which shift initiation does not occur. Due to the influence of noise, it sometimes happens that an incorrect determination of the engine speed occurs and leads to an improper determination of the shift initiation timing. This of course deteriorates the accuracy of any control which utilizes the timing of this event as a control parameter.